odhcpd: remove the "ra_management" option

odhcpd: remove the "ra_management" option

The option is marked as deprecated since 2019 in the odhcpd code base.

It has been superseded by the "ra_flags" and "ra_slaac" options, which provide
more fine-grained control, and having several different configuration options
controlling the same thing is a potential source of confusion.

The option is, and has always been, undocumented.

Furthermore, I've grepped through the LuCI source tree, and it contains logic
to control "ra_flags" and "ra_slaac", but no references to "ra_management" at
all.

The only references in the openwrt tree is inside the dnsmasq init script, but
that is a dnsmasq-internal thing and, again, the option cannot be set from
LuCI, meaning I think we can assume that it has limited usage.

So, remove it.

Signed-off-by: David Härdeman <[email protected]>
Link: https://github.com/openwrt/odhcpd/pull/294
Signed-off-by: Álvaro Fernández Rojas <[email protected]>